BPDCN

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Blastic plasmacytoid dendritic cell neoplasm (BPDCN)

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Morphologically the primitive cells may resemble monocytic AML, or may have less easy to define or even lymphoic morphology. The cells typically lie in the "blast area" - with dim to moderate CD45 expression with low side scatter. A skin rash is present in most cases.

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Potential difficulties in diagnosis:

The most common source of confusion is with the diagnosis of monoblastic AML, although cells may resemble lymphoid blast cells or may contain granules:

• Identifying primitive phenotype: CD7 and TdT are frequently expressed but CD34 is most often absent
• Definite lineage assignment: MPO, CD19, CD20, cCD22, CD79a and cCD3 should not be expressed
• BPDCN is a myeloid-derived neoplasm so expression of CD33 is relatively common, CD117 may also be expressed in some cases and rarely CD13 - this allows some cases to potentially fit the criteria for an AML diagnosis unless features of BPCDN are actively sought. Beware of cases with only two myeloid markers where there is skin infiltration, absent MPO and CD34

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What to look for:

Bright expression of: CD56 is generally accompanied by CD36, CD38, CD43, CD71, and HLA-DR, To confirm the diagnosis look also for plasmacytoid dendritic cell (PDC) associated markers: CD123, CD303, CD304, TCF4, TCL1 .

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MAJOR CRITERIA MINOR CRITERIA EXCEPTIONS

WHO diagnostic criteria allow BPDCN to be diagnosed:

• in the presence of CD123 and one other PDC-associated marker in addition to CD4 and/or CD56,
• in the presence of three PDC-associated markers and the absence of CD34 and other cell type-specific markers including CD3, CD14, CD19, lysozyme, and MPO.

Given the presence of other hematologic malignancies with similar phenotypes, correlation with histomorphology, clinical information and IHC studies is always necessary to make a definitive diagnosis of BPDCN.